The excellent exposure of the Higher Himalayan Crystallines (HHC) in Western Nepal allowed to constrain the tectonic, metamorphic and geochronological evolution of previously unrecognized high-temperature shear zones inside the HHC. A top-to-the SW high-temperature shear zone has been detected (Toijem Shear Zone: TSZ) in the middle portion of the HHC, bracketed between the Main Central Thurst to the bottom and the South Tibetan Detachment System to the top. In the TSZ mylonites abundant prismatic sillimanite crystals are aligned along the main foliation and wrap garnet and kyanite porphyroblasts. Abundant muscovite overgrows both kyanite and sillimanite crystals. Thermo barometric data indicate prograde metamorphism, in the kyanite field followed by decompression in the sillimanite field. U-Pb monazite ages on micaschists and gneisses, from both the footwall and the hanging wall of the TSZ, span between ca. 43 and 17 Ma, with major peaks at ca. 43, 38 and 33 Ma. The oldest age corresponds to monazite included in garnet in kyanite bearing gneiss, and is possibly recording the earliest stages of deformation and metamorphism of the HHC. Further monazite formation is attributed to continuing metamorphism and/or fluid activity. U-Pb ages from zoned monazites in sillimanite bearing mylonites of the TSZ indicate continuous or multiple metamorphism with a major episode at around 26 Ma. The ca. 43, 38 and 33 Ma monazite formation can be attributed to the underthrusting event and only later, from 26 Ma, exhumation has been active. The 26 Ma age, has been found only in the monazites on the TSZ mylonites and does not appear in previous geochronological studies of the Himalayan belt. The ca. 26 Ma could be attributed to the beginning of the TSZ activity marking the earliest stage of decompression of the HHC. Geometric, kinematic, geochronological and petrological data indicate that, after the underthrusting phase, the HHC in Dolpo area recorded the activity of a high-temperature shear zone (TSZ) developed during the earlier stages of exhumation of the HHC enhancing the decompression of the upper portion of the HHC before the main extrusion of the HHC by contemporaneous activity of the MCT and STDS constrained at 23-17 Ma.
Strain localization in the Higher Himalayan Crystallines: Geochronological constraints on shear zones activity before the Main Central Thrust in Western Nepal
MONTOMOLI, CHIARA;
2007-01-01
Abstract
The excellent exposure of the Higher Himalayan Crystallines (HHC) in Western Nepal allowed to constrain the tectonic, metamorphic and geochronological evolution of previously unrecognized high-temperature shear zones inside the HHC. A top-to-the SW high-temperature shear zone has been detected (Toijem Shear Zone: TSZ) in the middle portion of the HHC, bracketed between the Main Central Thurst to the bottom and the South Tibetan Detachment System to the top. In the TSZ mylonites abundant prismatic sillimanite crystals are aligned along the main foliation and wrap garnet and kyanite porphyroblasts. Abundant muscovite overgrows both kyanite and sillimanite crystals. Thermo barometric data indicate prograde metamorphism, in the kyanite field followed by decompression in the sillimanite field. U-Pb monazite ages on micaschists and gneisses, from both the footwall and the hanging wall of the TSZ, span between ca. 43 and 17 Ma, with major peaks at ca. 43, 38 and 33 Ma. The oldest age corresponds to monazite included in garnet in kyanite bearing gneiss, and is possibly recording the earliest stages of deformation and metamorphism of the HHC. Further monazite formation is attributed to continuing metamorphism and/or fluid activity. U-Pb ages from zoned monazites in sillimanite bearing mylonites of the TSZ indicate continuous or multiple metamorphism with a major episode at around 26 Ma. The ca. 43, 38 and 33 Ma monazite formation can be attributed to the underthrusting event and only later, from 26 Ma, exhumation has been active. The 26 Ma age, has been found only in the monazites on the TSZ mylonites and does not appear in previous geochronological studies of the Himalayan belt. The ca. 26 Ma could be attributed to the beginning of the TSZ activity marking the earliest stage of decompression of the HHC. Geometric, kinematic, geochronological and petrological data indicate that, after the underthrusting phase, the HHC in Dolpo area recorded the activity of a high-temperature shear zone (TSZ) developed during the earlier stages of exhumation of the HHC enhancing the decompression of the upper portion of the HHC before the main extrusion of the HHC by contemporaneous activity of the MCT and STDS constrained at 23-17 Ma.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
